CHAPTER 8
BASIC HYDRAULICS
hydraulic systems using seal materials compatible with
INTRODUCTION
petroleum-based fluids. The primary use for hydraulic
All modern naval aircraft contain hydraulic
fluid MIL-H-46170 is as a preservative fluid for
systems that operate various mechanisms. The number
hydraulic systems and components storage.
of hydraulically operated units depends upon the model
of aircraft. The average operational aircraft has about a
MIL-H-83282
dozen hydraulically operated units. Aircraft hydraulic
systems are designed to produce and maintain a given
MIL-H-83282 is the principal hydraulic fluid
pressure over the entire range of required fluid
used in military aircraft. MIL-H-83282 replaces
flow rates. The pressure used in most Navy
MIL-H-5606. It is dyed red so it can be distinguished
high-performance aircraft is 3,000 psi. The primary use
from incompatible fluids. MIL-H-83282 has a
of hydraulic fluids in aircraft hydraulic systems is to
synthetic hydrocarbon base and contains additives to
transmit power, but hydraulic systems perform other
provide the required viscosity and antiwear
functions. Hydraulic fluid acts as a lubricant to reduce
characteristics, which inhibit oxidation and corrosion.
friction and wear. Hydraulic fluid serves as a coolant to
It is used in hydraulic systems having a temperature
maintain operating temperatures within limits of
range of -40F to +275F. Flash point, fire point, and
critical sealant materials, and it serves as a corrosion
spontaneous ignition temperature of MIL-H-83282,
and rust inhibitor. Critical functions of hydraulic
which is fire resistant, exceeds that of MIL-H-5606 by
systems may be impaired if the hydraulic system fluid
more than 200F. The fluid extinguishes itself when the
is allowed to become contaminated beyond acceptable
external source of flame or heat is removed. Hydraulic
limits.
fluid MIL-H-83282 is compatible with all materials
Hydraulic fluid contamination is defined as any
used in systems presently using MIL-H-5606. It may be
foreign material or substance whose presence in the
combined with MIL-H-5606 with no adverse effect
fluid is capable of adversely affecting the system
other than a reduction of its fire-resistant properties.
performance or reliability. Contamination is always
MIL-H-83282 is now required in the main systems
present to some degree, even in new, unused fluid.
of all fleet aircraft previously using MIL-H-5606.
Contamination must be below the level that adversely
MIL-H-83282 is not used in some viscous dampers due
affects system operation. Hydraulic contamination
to its low-temperature characteristics.
control consists of requirements, techniques, and
practices that minimize and control fluid con-
MIL-H-5606
tamination. Remember the proverb, "An ounce of
prevention is worth a pound of cure."
MIL-H-5606 was the principal hydraulic fluid used
in naval aircraft before MIL-H-83282 was introduced.
MIL-H-5606 consists of petroleum products with
HYDRAULIC FLUIDS
additive materials to improve viscosity (temperature
LEARNING OBJECTIVE: Identify the
characteristics), inhibit oxidation, and act as an
types of hydraulic fluid used in naval aircraft
antiwear agent. The oxidation inhibiter was included to
and support equipment and their characteris-
reduce the amount of oxidation that occurs in
tics.
petroleum-based fluids when they are subjected to high
pressure and high temperature, and to minimize
Aircraft hydraulic systems are capable of reliable
corrosion of metal parts due to oxidation and resulting
unattended operation for long periods of time, but some
acids. The temperature range of MIL-H-5606 is
periodic service is generally required. Such service will
between -65F to +275F. It is dyed red so it can be
be either fluid servicing or air bleeding. Hydraulic
distinguished from incompatible fluids. Hydraulic
fluids MIL-H-5606, MIL-H-83282, and MIL-H-81019
fluid MIL-H-5606 is compatible with hydraulic
are used in automatic pilots, shock absorbers, brakes,
fluid MIL-H-46170.
control mechanisms, servo control systems, and other
8-1
